A typical motor vehicle air-conditioning system includes an evaporator located in the HVAC module adjacent to the passenger compartment and a condenser located in the front engine compartment exposed to outside ambient air. A compressor circulates a two-phase refrigerant through the evaporator where it expands into a low pressure vapor refrigerant by absorbing heat from the passenger compartment, thereby providing cooled air to the occupants. The low pressure vapor is compressed to a high pressure vapor by a compressor and then conveyed to the condenser where the high pressure vapor is condensed into a high pressure liquid refrigerant by releasing the heat to the ambient air. The liquid phase is returned to the evaporator through an expansion device which converts the high pressure liquid refrigerant to a low pressure mixture of liquid and vapor refrigerant to continue the cycle.
For vehicles having smaller internal combustion engines and for electric vehicles, it is known to operate the air-conditioning system in heat pump mode by reversing the refrigerant flow to provide supplementary heat to the passenger compartment. When the refrigerant flow is reversed, the condenser absorbs heat from the outside ambient air by evaporating the liquid phase refrigerant and the evaporator releases the heat to the passenger compartment by condensing the vapor phase refrigerant.
One disadvantage to operating the air-conditioning system in heat pump mode, since the low pressure side of the system when used in air conditioning mode would become the high pressure side when used in heat pump mode, is the increase in system complexity due to the requirement of having to reinforce the refrigerant plumbing throughout the system by using thicker gage tubing and fittings. Another known disadvantage of operating the system in heat pump mode is that in cooler climates, as the surface temperature of the condenser drop below 32° F., any moisture condensed on the surface of the condenser is subject to freezing, therefore resulting in reduced efficiency of the system and even potentially damaging the condenser.
Based on the foregoing, there is need for a heating system that provides supplementary heat to the passenger compartment of a motor vehicle that does not require reversing the refrigerant cycle of the air-conditioning system or detrimentally impacting the electric driving range.